1. Field of the Invention
The present invention relates to gas turbine engines and more particularly to vanes for use in engines having high turbine inlet temperatures.
2. Description of the Prior Art
The design and construction of gas turbine engines has always required precise engineering to ensure the structural integrity of the individual components. One particularly critical area for concern is the turbine nozzle which includes a plurality of vanes disposed across the flowpath of the high temperature gases which are discharged from the combustion chamber during operation. The side walls of adjacent vanes form a plurality of individual nozzles through which the hot gases flow. The flowing gases are directed circumferentially by the nozzles onto the blades of a rotating turbine wheel. The temperature of the combustion gases in the vicinity of the vanes normally exceeds the allowable temperature limit of the material from which the vanes are fabricated. Consequently, the vanes are cooled to reduce the operating metal temperature and prolong their service life.
Cooling air to the vanes is supplied from the compressor section of the engine. Commonly, compressor exit air is flowed from a port at the inner diameter of the working medium gas path through various conduit means to the turbine section of the engine. Each vane commonly has a hollow cavity within the airfoil section which receives the cooling air. A typical vane utilized in cooled turbines is shown in U.S. Pat. No. 3,628,880 to Smuland et al. In Smuland a baffle is inserted into a hollow cavity at the leading edge of a vane airfoil section. Cooling air is directed by small diameter holes in the baffle to impinge upon the cavity walls and, subsequently, is flowed over the outer walls of the airfoil section via leading edge holes in the airfoil wall to film cool the outer surfaces of the vane.
Film cooling requires a precise but relatively low pressure drop across the flow emitting holes at the leading edge. If the pressure drop is too great the emitted flow penetrates the passing working medium and is deflected downstream with the combustion gases without establishing a film layer on the airfoil surface. On the other hand if the pressure drop is too small the hot combustion gases penetrate the cooling air layer to cause destructive heating of the vane material. Impingement cooling, however, requires a high pressure across the baffle insert in order to accelerate the flow to impinging velocities at the airfoil section wall. In order to establish the required baffle pressure drop, the pressure within the hollow cavity must be significantly higher than the pressure of the working medium to which the impinging flow is exhausted.
To implement the conjunctive use of impingement and film cooling, continuing efforts are being directed to provide apparatus which will isolate cooling air to the leading edge holes from the exhaust holes to the working medium for the impingement flow.